How to make a "simple" GET request if the resource doesn't allow CORS?

I'm trying to use W3C's CSS Validator Web Service API in a Chrome new tab extension. The docs claim that a request is a simple HTTP GET call to a URI, so I figured this should work:

fetch('http://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fwww.w3.org%2F&profile=css3&output=json')
.then((response) => {
  // do stuff with response
});

Unfortunately the promise fails and I get a message like:

Failed to load. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin is therefore not allowed access.

How am I supposed to make a "simple" GET request if the resource doesn't allow CORS?

Thanks!

Solution

2017-10-22 Update: The change for adding CORS support to the CSS Validator was merged and pushed to production, so the following now works as-is:

const validatorURL = 'https://jigsaw.w3.org/css-validator/validator' +
  '?uri=http%3A%2F%2Fwww.w3.org%2F&profile=css3&output=json';
fetch(validatorURL)
  .then(response => response.json())
  .then(results => console.log(results))

Original answer:

The CSS Validator doesn’t send the necessary CORS response headers. To fix that, I’ve raised a pull request with a change against the CSS Validator sources that’ll enable your code snippet to work as-is. I’ll post an update here once the change is merged and pushed to production.

In the meantime, you can work around the issue by making your request through a CORS proxy:

const proxyURL = 'https://cors-anywhere.herokuapp.com/';
const validatorURL = 'http://jigsaw.w3.org/css-validator/validator' +
  '?uri=http%3A%2F%2Fwww.w3.org%2F&profile=css3&output=json';
fetch(proxyURL + validatorURL)
  .then(response => response.json())
  .then(results => console.log(results))

For details abou why that works, see the How to use a CORS proxy to get around “No Access-Control-Allow-Origin header” problems section of the answer at No 'Access-Control-Allow-Origin' header is present on the requested resource—when trying to get data from a REST API.